(1) Field of the Invention
This invention is concerned with impact printing devices such as typewriters and, in particular, with such mechanisms in which the printing is achieved by a rotary device such as a daisy-type wheel or a cup-shaped printing element. Such elements typically include a plurality of spokes or petals each bearing a character or characters thereon.
A printing element of this type is commonly located on a carrier for movement along a print line direction during the printing operation. As the carrier is moved from print position to print position along the print line direction, the printing element is rotated about its axis for character selection purposes. When the printing element is in its proper position (i.e. when the character on the printing element to be printed is opposite the desired print position), the carrier is momentarily stopped and a print hammer on the carrier impresses the character on the printing element against an inking ribbon and a print receiving medium, such as paper.
After a printing operation occurs, the print hammer returns to its rest position and the printing element is rotated so that the next character on the printing element to be printed is opposite the desired print position. The next, and subsequent, printing steps occur in the same manner as described above, until a complete line has been printed. After printing a complete line, appropriate means are provided to move the carrier to the next starting line position for subsequent printing. A printing device which operates in this manner is the typewriter sold by SCM Corporation under the model "XE5000."
In printing devices of this type, it is essential that means be provided to assure that the printing element is engaged and maintained in its home position after the power switch to the printing device is turned on. Furthermore, it is highly desirable that the maintenance of the printing element in its home position be achieved without any accompanying noise. The purpose of the present invention is to provide such means.
(2) Description of the Prior Art
Examples of prior art patents which disclose printing element homing devices include U.S. Pat. No. 4,264,220 issued to S. A. Okcuoglu on Apr. 28, 1981 for a "Print Wheel Homing Apparatus"; U.S. Pat. No. 4,299,502 issued to K. J. S. Harre on Nov. 10, 1981 for an "Assembly for a Detachable Connection for a Printing Element in an Electric Office Machine"; and U.S. Pat. No. 4,428,694 issued to R. A. Ragen on Jan. 31, 1984 for a "Rotary Printing Device with Identifying Means and Method and Apparatus for in Situ Identification."
The Okeuoglu patent discloses a mechanism to assure that a rotary printing element is in its home position. Means are provided which first shift the print wheel carrier to an extreme position on the printer frame wherein a stop element is mounted. When the carrier is in its extreme position, the stop element extends into the path of another stop element attached and rotatable with the printing element. The printing element is then rotated until the stop element on the printing element engages the stop element on the printer frame. That position of engagement defines the home position of the printing elements.
The Harre patent discloses a means of removing a print element from an electric office machine that requires the printing element to be in its home position. To locate the print element in its home position, the print element is manually rotated until a dog member engages a recess in a lock washer connected to the print element.
The patent to Ragen discloses a means for identifying the "home" position of a printing device. That is accomplished by providing a protrusion on the print wheel which is rotatable therewith. An interposer abuts the rotatable protrusion and stops the print wheel drive motor to identify the home position. The information concerning the home position of the print wheel is then furnished to a microprocessor in the printing device.